This invention relates to the pumping of liquids which would vaporise at normal temperature and pressure, hereinafter referred to as NTP. Such liquids therefore have to be stored, and pumped, at low temperatures and/or at pressures which are suitably greater than atmospheric. The invention is particularly applicable to, but is not restricted to, the pumping of liquefied natural gas, hereinafter referred to as LNG. It is frequently necessary to pump LNG from a tank to a receiver system, e.g. from a sealed tank of a marine vessel to a header leading to a shore installation. A conventional installation for this purpose commonly comprises a pump, such as a centrifugal pump, which is wholly immersed in the liquid in the tank. From the outlet of the pump a riser pipe rises to a point above the level of the liquid in the tank, and emerges externally of the upper part of the tank, e.g. the usual tank dome. A stop valve is fitted to the riser pipe, and a connection is made from the stop valve, e.g. through a flexible connection, to a header leading to a shore installation.
With the stop valve closed, and the system generally static, there will be a level of liquid in the riser pipe which is substantially the same as the level of the remainder of the liquid in the tank. The space within the riser pipe above the liquid level will be filled with the LNG vapor. If the pump is then operated, with the stop valve closed, the body of liquid accelerating up the riser pipe compresses the vapor and turns it to liquid. Accordingly, the advancing column of liquid reaches, and is substantially instantaneously checked by the stop valve. This abrupt impact is known in the art as valve "slam", and it can damage or destroy the stop valve or piping.